1. Field of the Invention
The invention relates to a process for preparing silicone-containing copolymers of an ethylenically unsaturated organic monomer and a silicone macromer in the form of their aqueous polymer dispersions or water-redispersible polymer powders.
2. Description of the Related Art
Various methods of preparing polymer dispersions of silicone-modified copolymers are known from the prior art. In EP-A 1308468, such copolymers are prepared by a process in which the total amount of silicone and a partial amount of the monomers are initially charged in an aqueous emulsion and are polymerized in the presence of a water-soluble initiator. EP-A 352339 describes a solution polymerization process in which the silicone component is initially charged in the solvent and a mixture of monomers and oil-soluble initiator is continuously metered in. EP-A 771826 discloses a process for preparing a crosslinked silicone copolymer latex, in which water, monomer, emulsifier and water-soluble initiator are initially charged, the reaction is started, further monomer is slowly metered in and the crosslinking silicone, which has a short chain and is multiply unsaturated, is finally added together with the remaining monomer.
In EP-A 614924, only short-chain silicone macromers are used in the emulsion polymerization, since relatively long-chain silicone macromers do not polymerize sensibly with the organic monomers. U.S. Pat. No. 6,602,949 describes the preparation of silicone-organopolymer graft polymers, in which a branched, short-chain silicone having a dendritic structure is reacted with an ethylenically unsaturated radical, ethylenically unsaturated monomers and a free-radically polymerizable emulsifier in the presence of an oil-soluble initiator. The dendritic structure of the short-chain silicone macromer improves the copolymerization with organic monomers. It is shown in comparative examples that long-chain silicone macromers (without a dendritic structure) have polymerized to an extent of not more than 75% with organic monomers and a large amount of unreacted silicone macromer remains. In EP-A 810243 and JP-A 05-009248, silicone macromers are polymerized with organic monomers in emulsion, exclusively by means of an oil-soluble initiator. A disadvantage of the processes initiated by means of an oil-soluble initiator is the unsatisfactory stability of the resulting dispersions, which display a strong tendency to undergo phase separation.
U.S. Pat. No. 5,618,879 describes the copolymerization of a mixture of silicone macromer and monomer which is emulsified in water by means of an anionic emulsifier, with the polymerization being initiated by means of a water-soluble initiator. In JP-A 05-140255, the silicone macromer is dissolved in the organic monomer, is emulsified in water by means of an anionic emulsifier and the polymerization is initiated by means of a water-soluble initiator. Here too, a disadvantage is that a considerable proportion of over 20% of the silicone macromer is not copolymerized.
In all the processes known from the prior art, the copolymerization of the silicone macromers with organic monomers in emulsion always proceeds only to an unsatisfactory extent. This leads to free silicone remaining in the dispersion, with corresponding disadvantageous consequences: the silicone migrates out of coatings or films; the dispersion can coagulate; and the particle size distribution is inhomogeneous. In addition, the tendency to undergo phase separation has an adverse effect on the storage stability.